A lot of semiconductive rollers are incorporated in an image forming apparatus. More specifically, semiconductive rollers each produced by forming a semiconductive rubber composition into a roller shape and crosslinking the rubber composition are generally used as a charging roller for uniformly electrically charging a surface of a photoreceptor body, as a developing roller for developing an electrostatic latent image formed by light-exposing the electrically charged photoreceptor surface into a toner image, as a transfer roller for transferring the formed toner image onto a paper sheet or the like, and as a cleaning roller for removing toner from the photoreceptor surface after the transfer of the toner image to the sheet.
Such a semiconductive roller is used with a shaft of a metal or the like inserted through and fixed to a center through-hole of the roller.
The rubber composition as a material for the semiconductive roller is generally imparted with ion conductivity by blending an ion conductive rubber such as an epichlorohydrin rubber as a rubber component, or imparted with electron conductivity by blending an electrically conductive carbon black or the like.
Further, it is a general practice to use a diene rubber in combination with the ion conductive rubber as the rubber component for the rubber composition to improve the mechanical strength and the durability of the semiconductive roller and to impart the semiconductive roller with rubber characteristic properties, i.e., to make the semiconductive roller flexible and less susceptible to permanent compressive deformation with a reduced compression set.
Incidentally, minute particles such as of silica and titanium oxide are externally added as external additives to a toner for use in the image forming apparatus in order to control the fluidity, the chargeability and other characteristic properties of the toner.
However, these external additives and finely broken toner particles occurring when image formation is repeated (hereinafter referred to simply as “external additives”) cannot be perfectly removed from the photoreceptor surface by means of a cleaning blade and the like. It is known that, as the image formation is repeated, the external additives left unremoved are liable to adhere to an outer peripheral surface of the semiconductive roller incorporated in the image forming apparatus, particularly to an outer peripheral surface of a charging roller constantly kept in contact with the photoreceptor surface, to be thereby gradually accumulated on the roller.
The external additives accumulated on the semiconductive roller are liable to influence the photoreceptor chargeability or to adhere to a formed image to cause an image defect.
Particularly, where the semiconductive roller is produced by forming the semiconductive rubber composition into a tubular body, crosslinking the tubular body and polishing an outer peripheral surface of the tubular body to a predetermined surface roughness, the outer peripheral surface has minute undulated polishing marks. When the image formation is repeated, the external additives are liable to adhere to and accumulated in the polishing marks, thereby causing the aforementioned problems.
In order to suppress the adhesion of the external additives, it is conceivable to coat the outer peripheral surface of the semiconductive roller with a coating film such as of a urethane resin to improve the slipperiness of the outer peripheral surface with respect to the toner and the external additives.
However, the coating film is generally formed by applying a coating agent onto the outer peripheral surface of the semiconductive roller by a spraying method, a dipping method or the like and then drying the applied coating agent. Therefore, the coating film is liable to suffer from contamination with dust and other foreign matter during the formation thereof, thickness unevenness and other defects.
With the aforementioned defects, the semiconductive roller serving as the charging roller fails to uniformly electrically charge the surface of the photoreceptor body, thereby causing an image defect such as image density unevenness.
For preparation of the coating agent, an organic solvent is required. The use of the organic solvent may exert a great load on the environment, and go against a recent trend toward reduction of VOC (volatile organic compounds).
It is also contemplated to form a multiplicity of independent concavities each having a significantly greater size than the polishing marks in the outer peripheral surface of the semiconductive roller, for example, by a laser processing, rather than forming the coating film (see, for example, Patent Documents 1 to 3).
In this case, the minute polishing marks are removed by the formation of the concavities. Further, the surface roughness and the electrical properties of the outer peripheral surface are made uniform, and the contact area and the contact pressure of the outer peripheral surface with respect to the surface of the photoreceptor body for the image formation can be controlled by arraying the multiplicity of concavities in the outer peripheral surface. Thus, it is expected to alleviate stresses on the outer peripheral surface and the toner during the image formation, thereby suppressing the adhesion and the accumulation of the external additives and the associated problems.